Purealin, a novel activator of skeletal muscle actomyosin ATPase and myosin EDTA-ATPase that enhanced the superprecipitation of actomyosin

Bromo-spiroisoxazoline Alkaloids, Including an Isoserine Peptide, from the Caribbean Marine Sponge Aplysina lacunosa

Three new bromotyrosine spiroisoxazoline alkaloids, lacunosins A and B (1 and 2) and desaminopurealin (3), were isolated from a MeOH extract of the marine sponge Aplysina lacunosa that showed modest α-chymotrypsin inhibitory activity. The structures of 1-3 share the spirocyclohexadienyl-isoxazoline ring system found in purealidin-R and several other Verongid sponge secondary metabolites. Compounds 1 and 2 are coupled to a glycine and an isoserine methyl ester, respectively. Alkaloid 3 is linked, contiguously, to an O-1-aminopropyl 3,5-dibromotyrosyl ether and, finally, to histamine through an amide bond. The planar structures of all three compounds were obtained from analysis of MS and 1D and 2D NMR data. The absolute configuration of the SIO unit of 1-3 was assigned by electronic circular dichroism (ECD). The isoserine amino acid residue in 2 was found to be a 1:1 mixture of epimers using a new Marfey's type reagent, derived from Trp-NH2. Allylic O-naphthoylation of the SIO subunit enhances the ECD spectrum of SIOs and improves discrimination of enantiomorphs. A unifying hypothesis is proposed that links the biosynthesis of several of the new compounds with previously reported analogues.

Unfolding of the myosin head by purealin in glycerol

Purealin is a small bioactive compound obtained from the marine sponge. The compound modulates various types of ATPase activity of myosin from skeletal muscle, cardiac muscle, and smooth muscle. To elucidate the structural basis of these effects of purealin on myosin ATPases, we examined the effect of purealin on the conformation of skeletal muscle myosin in aqueous solution and in glycerol. Analysis of the circular dichroism spectrum of subfragment 1, a single-headed fragment of myosin, revealed that in 10% glycerol purealin decreased the β-sheet content of S1, but in aqueous solution it had little effect on the secondary structure of S1. A myosin monomer conforms to two pear-like globular heads attached to a long tail. Electron microscopy observations with rotary shadowing revealed that purealin unfolded each globular head to an extended single strand. The tips of the unfolded strand bound each other and formed a ring in one molecule. These results suggest that binding of purealin affects the critical parameters of myosin folding.

Synthesis and biological evaluation of purealin and analogues as cytoplasmic dynein heavy chain inhibitors

Cytoplasmic dynein plays important roles in membrane transport, mitosis, and other cellular processes. A few small-molecule inhibitors of cytoplasmic dynein have been identified. We report here the first synthesis of purealin, a natural product isolated from the sea sponge Psammaplysilla purea, which is known to inhibit axonemal dynein. Also described are the first syntheses, by modular amide coupling reactions, of the natural product purealidin A (a component of purealin) and a small library of analogues. The library was examined for inhibition of cytoplasmic dynein heavy chain and cell growth. The compounds showed effective antiproliferative activity against a mouse leukemia cell line but selective activities against human carcinoma cell lines. Purealin and some of the analogues inhibited the microtubule-stimulated ATPase activity of recombinant cytoplasmic dynein heavy chain motor domain. The inhibitory effect of purealin was concentration dependent and uncompetitive, supporting the hypothesis that it does not compete with the binding of ATP.

Halenaquinone, a novel phosphatidylinositol 3-kinase inhibitor from a marine sponge, induces apoptosis in PC12 cells

In nerve growth factor-treated PC12 cells, 12b-methyl-(S)-1H-benzo[6,7]phenanthro[10,1-bc]furan-3,6,8,11(2H,12bH)-tetrone (halenaquinone) caused cytotoxicity in a concentration-dependent manner (EC(50) value; 10 microM). Gel electrophoretic DNA analysis of PC12 cells treated with halenaquinone (10 microM) and 11-(acetyloxy)-1,6b,7,8,9a,10,11,11b-octahydro-1-(methoxymethyl)-9a,11b-dimethyl-[1S-(1 alpha,6b alpha,9a beta,11 alpha,11b beta)]-3H-furo[4,3,2-de]indeno[4,5-h]-2-benzopyran-3,6,9-trione (wortmannin) (3 microM) showed a typical apoptotic DNA ladder. In the flow cytometric analysis, halenaquinone caused apoptosis in a concentration- and time-dependent manner (EC(50) value; 10 microM), whereas 2,3-dihydro-12b-methyl-(S)-1H-benzo[6,7]phenanthro[10,1-bc]furan-6,8,11(12bH)-trione (xestoquinone) with the methylene group at the C-3 position failed to cause apoptosis, suggesting that the carbonyl group at the C-3 position in halenaquinone is important for exerting apoptotic effects in PC12 cells. Phosphatidylinositol 3-kinase was inhibited by halenaquinone (IC(50) value; 3 microM) as well as wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase. Halenaquinone inhibited phosphatidylinositol 3-kinase activity at lower concentrations than those at which it induced apoptosis in PC12 cells. These results suggest that halenaquinone causes the death of PC12 cells through an apoptotic process and that the mechanism of halenaquinone-induced apoptosis may be partially explained by the inhibition of phosphatidylinositol 3-kinase activity.

Amphidinolide B, a powerful activator of actomyosin ATPase enhances skeletal muscle contraction

Amphidinolide B caused a concentration-dependent increase in the contractile force of skeletal muscle skinned fibers. The concentration-contractile response curve for external Ca2+ was shifted to the left in a parallel manner, suggesting an increase in Ca2+ sensitivity. Amphidinolide B stimulated the superprecipitation of natural actomyosin. The maximum response of natural actomyosin to Ca2+ in superprecipitation was enhanced by it. Amphidinolide B increased the ATPase activity of myofibrils and natural actomyosin. The ATPase activity of actomyosin reconstituted from actin and myosin was enhanced in a concentration-dependent manner in the presence or absence of troponin-tropomyosin complex. Ca2+-, K+-EDTA- or Mg2+-ATPase of myosin was not affected by amphidinolide B. These results suggest that amphidinolide B enhances an interaction of actin and myosin directly and increases Ca2+ sensitivity of the contractile apparatus mediated through troponin-tropomyosin system, resulting in an increase in the ATPase activity of actomyosin and thus enhances the contractile response of myofilament.

Modulation of actomyosin ATPase by goniodomin A differs in types of cardiac myosin

Goniodomin A causes the conformational change of actin to modify actomyosin ATPase activity [Furukawa, K.-I., Sakai, K., Watanabe, S., Maruyama, K., Murakami, M., Yamaguchi, K., Ohizumi, Y., 1993. Goniodomin A induces modulation of actomyosin ATPase activity mediated through conformational change of actin. J. Biol. Chem. 268, 26026-26031]. Goniodomin A inhibited the ATPase activities of atrial myofibrils, myosin B and reconstituted actomyosin in a concentration-dependent manner. Interestingly, these ATPase activities of ventricular muscle were enhanced by goniodomin A (3 x 10(-8)-3 x 10(-7) M), but were decreased when the concentration was further raised. The stimulatory effect of goniodomin A was significantly inhibited by troponin tropomyosin complex. These results suggest that goniodomin A affects actin to modify cardiac actomyosin ATPase activity, and that this modulation differs in types of cardiac myosin.

Purealin blocks the sliding movement of sea urchin flagellar axonemes by selective inhibition of half the ATPase activity of axonemal dyneins

Ciliary and flagellar movements are explained by active sliding between the outer doublet microtubules of an axoneme via their inner and outer dynein arms. Purealin, a novel bioactive principle of a sea sponge Psammaplysilla purea, blocked the motility of Triton-demembranated sea urchin sperm flagella within 5 min at concentrations above 20 microM. In a similar concentration range, purealin blocked the sliding movement of the flagellar axonemes in vitro within a few minutes judging from the turbidity measurements. The ATPase activity of axonemes was partially inhibited by purealin in a concentration-dependent manner. The maximum inhibition reached approximately 50% at concentrations above 20 microM, indicating that half the axonemal ATPase activity is sensitive to purealin. Similar results were observed on the ATPase activity of outer-arm-depleted axonemes and that of a mixture of 21S dynein and salt-extracted axonemes. On the other hand, ATPase activity of isolated 21S dynein was not inhibited by purealin. The inhibitory action of purealin on the axonemal ATPases was reversed by dilution of purealin. The effect of purealin on the double-reciprocal plot of the ATPase activity as a function of ATP concentrations showed that the inhibition was not a competitive type. In accord with this finding, purealin did not affect the vanadate-mediated UV photocleavage of axonemal dyneins. These results suggest that purealin binds reversibly to a site other than the catalytic ATP-binding site and inhibits half the ATPase activity of axonemes. Taken together, our results suggest that purealin-sensitive ATPase activity of the dynein arms plays an essential role in generating the sliding movement of flagellar axonemes.